1. Field of the Invention
The present invention relates to a video clock signal generator which receives a light emitted from a light source by a linear scale through a vibration scanner, detects a light reflected by or transmitted through a grating of the linear scale and processes an output signal thereof to generate a clock pulse, and more particularly to a video clock signal generator in an optical scan type image input device such as a laser scan microscope which scans a light irradiated to a sample and senses a reflected light, a transmitted light or a fluorescent light from the sample in accordance with a position of the light to form an image.
2. Related Background Art
An optical scan type image input device disclosed in U.S. Pat. No. 4,212,018 generates a video clock signal by using a linear scale. It is easy to realize and hard to be affected by disturbance.
In the prior art, a laser beam from a laser light source is focused and reflected by a mirror surface of a scanner, and the reflected light is directed onto a linear scale as a scanning light spot by the rotation of the scanner. The light spot is scanned to cross a grating of the linear scale. Since it crosses the grating as it is scanned, an intensity of the laser beam transmitting through the linear scale varies. The modulated laser beam is converted to a modulated signal by a photo-sensor and the modulated signal is comparated at an appropriate level so that a video clock signal is generated in accordance with the rotation of the scanner.
The prior art discloses the scanner which uses a polygon mirror although a scanner which reciprocally scans a light spot on a linear scale such as a galvanometer or a resonance type galvanometer (resonant mirror which vibrates at its resonance frequency) is also known.
Particularly, the resonant mirror is effective for the image input by high speed scan because of its high vibration frequency. However, since the resonance frequency varies by the affect of change in an environment such as a temperature, an amplitude is not stable even if an input frequency is constant, and it is difficult to form an ideal image.
In order to increase an image density, it is necessary to increase the number of video clock signals. In order to increase the video clock signals while keeping a swing angle of the scanner at a constant level, the size of the linear scale may be doubled and a focal distance of a focusing lens may be elongated to double the amplitude of the light spot. Alternatively, a pitch of the grating of the linear scale may be reduced to one half and a diameter of the light spot may be reduced to one half. However, when the focal distance of the focusing lens is elongated, the diameter of the light spot on the linear scale increases and a light spot which is small enough to the pitch of the grating cannot be attained. Same is true when the pitch of the grating is reduced to one half. In the prior art optical scan type image input device, there is a limit in the increase of the number of video block signals whatever scanner may be used.
In the prior art device, the frequency of the amplified photo-sensor output is multiplied by a PLL (phase-locked loop) circuit, and it is supplied to a comparator to generate a required number of clock signals. The number of video clock signals may be increased by increasing the frequency multiplication factor by the PLL circuit. Where the amplitude of the scanner and the scan velocity are constant, an enlarged image may be formed by this method. However, in order to increase the multiplication factor, it is necessary to use a high speed IC for the PLL circuit, and even with such a high speed IC, the circuit may not be compatible with a high frequency or the configuration of the PLL circuit is complex. As a result, it is not easy to attain an enlarged image.